iOS Background Tasks

AppDelegate.cs

using System;
using System.Diagnostics;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;
using Foundation;
using ModernHttpClient;
using UIKit;

namespace App6
{
    [Register("AppDelegate")]
    public partial class AppDelegate : UIApplicationDelegate
    {
        // class-level declarations
        UIWindow _window;

        public override bool FinishedLaunching(UIApplication app, NSDictionary options)
        {
            // create a new window instance based on the screen size
            _window = new UIWindow(UIScreen.MainScreen.Bounds);
            _window.MakeKeyAndVisible();

            app.RegisterUserNotificationSettings(
                UIUserNotificationSettings.GetSettingsForTypes(
                    UIUserNotificationType.Badge | UIUserNotificationType.Alert, null));

            return true;
        }

        private CancellationTokenSource _cancellationTokenSource;
        public override async void DidEnterBackground(UIApplication application)
        {
            var taskId = UIApplication.SharedApplication.BeginBackgroundTask(BackgroundTimeExpired);
            
            _cancellationTokenSource = new CancellationTokenSource();

            try
            {
                await TransferAsync(50, _cancellationTokenSource.Token);
            }
            catch (OperationCanceledException) {}
            catch (AggregateException) { }

            UIApplication.SharedApplication.EndBackgroundTask(taskId);
        }

        private void BackgroundTimeExpired()
        {
            if (_stopwatch.IsRunning)
                _stopwatch.Stop();

            _cancellationTokenSource.Cancel();

            var notification = new UILocalNotification
            {
                AlertAction = "Background Transfer",
                AlertBody = string.Format("Background Time Expired after: {0:mm} minutes", _stopwatch.Elapsed)
            };

            UIApplication.SharedApplication.PresentLocalNotificationNow(notification);
        }

        private Stopwatch _stopwatch;
        private int _iteration;
        private async Task TransferAsync(int iterations, CancellationToken token)
        {
            token.ThrowIfCancellationRequested();

            var url = "https://github.com/paulcbetts/ModernHttpClient/releases/download/2.2.0/ModernHttpClient-2.2.0.zip";
            var handler = new NativeMessageHandler();
            var client = new HttpClient(handler);
            _iteration = 0;

            _stopwatch = new Stopwatch();
            _stopwatch.Start();
            while (!token.IsCancellationRequested && _iteration < iterations)
            {
                var bytesRead = 0;
                try
                {
                    using(var request = new HttpRequestMessage(HttpMethod.Get, url))
                    using (var resp =
                        await client.SendAsync(request, HttpCompletionOption.ResponseHeadersRead, token))
                    {
                        var bytes = await resp.Content.ReadAsByteArrayAsync();
                        bytesRead = bytes.Length;

                        bytes = null;
                    }
                }
                catch { }

                PresentNotification(_iteration, bytesRead);
                _iteration++;

                await Task.Delay(6000, token);
            }
            _stopwatch.Stop();
        }

        private static void PresentNotification(int iteration, int bytesRead)
        {
            var notification = new UILocalNotification {
                AlertAction = "Background Transfer",
                AlertBody = string.Format("Fetched file: {0}, Bytes: {1}", iteration, bytesRead),
                ApplicationIconBadgeNumber = iteration
            };

            UIApplication.SharedApplication.PresentLocalNotificationNow(notification);
        }
    }
}